I've done AquaSeas, which is the version of this tour for non-scuba divers. Basically, you're in full scuba gear, but without any weights, so you stay on the surface, like snorkeling but with scuba breathing apparatus instead of a snorkel.
It was VERY cool and VERY worthwhile. We'd like to get our scuba cert someday, and when we do, we'll be back for AquaSeas!